1. Field of the Invention
The present invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response signaling and maximum-likelihood (PRML) detection together with digital filtering, and more particularly to apparatus and method for providing equalization adjustment for a filter in a PRML magnetic recording channel.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. A PRML channel can be used to achieve high data density in writing and reading digital data on the disks.
To obtain full advantage of the PRML channel, the received signal or the read signal must be passed through a specially designed equalizing filter which produces at its output a signal spectrum corresponding to the wave shape for which the maximum-likelihood detector is designed. When using digital filtering in a PRML system, the filter is located between an analog-to-digital converter (ADC) and other signal processing hardware which controls the system and performs the detection. Various equalizer adjustments have been implemented during data recovery in PRML to improve hard error rate.
U.S Pat. No. 5,258,940 issued Nov. 2, 1993 and assigned to the present assignee, discloses a distributed arithmetic (DA) digital filter having many taps, such as a ten 10-tap filter implemented in RAM. The disclosed DA digital filter is configured with an automatic equalization architecture that does not require an adaptive loop. A plurality of partial sums of predetermined tap weights are stored in a palette RAM. A fixed qualifier value is received and used together with selected ones of the stored partial sums for calculating a predetermined tap weight. Predetermined filter coefficients are calculated and stored in a filter RAM using the calculated predetermined tap weight and predefined ones of the stored partial sums.
U.S Pat. No. 5,268,848 issued Dec. 7, 1993 and assigned to the present assignee, discloses an apparatus and method for providing equalization adjustment for a finite impulse response (FIR) digital filter in a PRML data channel. A plurality of transformations are defined to transform effective tap weights in a digital filter into modified effective tap weight values by adding each initial effective tap weight value with a selected cosine equalization adjustment function. A cosine expander circuit is provided for expanding 8-byte compressed equalizations into a 32-byte format and for altering the filter response by a selected cosine equalization adjustment function.
Effective textbook solutions to the problem of equalization are available; however, these schemes do not lend themselves to a manufacturing environment because they require external hardware. Thus, a need exists to implement a robust equalization technique which uses available internal disk file hardware, with minimal test overhead on the manufacturing line. It is desirable to provide equalization algorithm that minimizes test time without compromising equalization integrity. An easily manufacturable method for finely tuned equalizations of a PRML channel to variations in magnetic components in a DASD is needed.